In the automatic testing of electrical circuits, test probes of various configurations are used, depending upon such factors as the type of electrical device under test, the spacing between test points, and the like. The present invention is applicable to many different types of contact test probes. In order to appreciate the improvements provided by this invention, one type of prior art test probe will first be described. This test probe assembly comprises a conventional spring-loaded contact probe generally including an outer receptacle, a movable plunger or probe member, and a barrel in the receptacle. In such devices the plunger or probe member is mounted in the barrel and extends outwardly from it. The barrel includes a spring so that the plunger or probe member is supported in the barrel by spring pressure. The probe member can be biased outwardly a selected distance by the spring and may be biased or depressed inwardly into the barrel a selected distance under force directed against the spring. The probe member generally has a probing end or tip which makes contact with selected test points on the electrical device under test. A printed circuit board would be an example of a unit under test.
There are many applications in which the probing end of the test probe must grip onto a particular circuit element or test point during probing. Therefore, the test probe must be designed so that it stays in good contact with the circuit test point being probed. In order to ensure good contact during probing, prior art test probes have included a large number of different probe tip configurations designed to grip onto the test point to make good contact when spring pressure is applied to the probe during testing.
Examples of instances in which good contact is required between the test probe and the circuit test point include where the board under test has been in a dirty environment, or where no de-fluxing was conducted after soldering; and therefore, the test probe must grip or bite into the conductive surface of the test point to ensure circuit continuity.
A conventional way of ensuring good probe contact is by use of a "helical" test probe assembly which includes a cylindrical barrel having an axial passage and a movable test probe which slides axially in the passage. The barrel has a coil spring inside it which applies a spring bias to a plunger during use. The plunger has a small lateral post extending outwardly and engaged in a spiral groove extending lengthwise along the barrel. The post engaged in the groove provides a means for controlling movement of the probe relative to the barrel during use. When the plunger portion of the probe assembly is depressed against the spring pressure, the post rides upwardly in the spiral groove and causes the probe shaft to rotate about its axis during probing. By twisting about its axis, the bottom contact tip portion of the probe can bite into the test point on the unit under test to provide better contact during testing.
In another approach, the post can be engaged in a linear groove to prevent the probe shaft from rotating about its axis during its linear travel. This can ensure better control over undesired lateral movement of the test probe during use and orient the plunger tip relative to the unit under test.
There are certain drawbacks to using a small lateral post and groove combination to control rotation of the probe. These disadvantages include greater cost of production and poor reliability during use, especially prolonged use involving many test cycles. The overall dimension of the test probe also is affected and limits application to close on-center spacing among test probes.
The present invention provides a test probe which, in one configuration referred to as a "helix probe", provides a means for controlling test probe motion during use, so that the contact tip portion of the probe rotates during testing for making good surface contact with the test point on the unit under test. Another embodiment, referred to as a "linear probe", provides a means for preventing rotation of the probe shaft relative to the barrel during linear travel of the probe shaft. Either embodiment of the invention avoids the disadvantages of the prior art use of a test probe with a post engaged in a groove in the barrel of the probe assembly for providing rotational control of the probe.